This invention relates to liquefied gas boilers and to methods of boiling liquefied gas, i.e., as defined herein, the liquid phase of a substance which has a boiling point of 20.degree. or below at 1 atmosphere absolute. More particularly, this invention relates to condenser-reboilers for use in association with air separation columns.
In a conventional double column for the separation of air (from which constituents of relatively low volatility such as carbon dioxide and water vapor have been removed) the lower column is operated at a relatively elevated pressure in comparison with the upper column. A condenser-reboiler condenses nitrogen vapor at the top of the lower column and reboils liquid oxygen at the bottom of the upper column. The condenser-reboiler thus provides a thermal link between the two columns, and in effect, given a predetermined operating pressure at the bottom of the upper column, determines the operating pressure and the temperature at the top of the lower column. In order to provide the necessary thermal energy to reboil the liquid oxygen, it is necessary that the nitrogen condense at a higher temperature than that of the boiling point of the liquid oxygen. The more efficient the heat exchange between the condensing nitrogen and the boiling liquid oxygen, the less the temperature difference between the two fluids in the condenser reboiler needs to be, and hence the lower the temperature and pressure at which the nitrogen condenses. Moreover, as a consequence of more efficient heat exchange and lower operating pressure in the lower column, less work need be done in compressing the air to the operating pressure of the lower column. Alternatively, the advantage of more efficient heat exchange can be reaped in employing a smaller condenser-reboiler.
The temperature difference between the temperature of the heated wall and the boiling liquid oxygen is defined by the quantity Q/hA where Q/A is the heat flux or heat flow per unit area absorbed in boiling the liquefied gas, A is the nominal surface area of the surface at which the liquefied gas is boiled and h is a quantity known as the boiling heat transfer co-efficient. Accordingly, for given values of Q and A, the temperature difference decreases with increases in the boiling heat transfer co-efficient. There are many proposals in the art for increasing the boiling heat transfer co-efficient of heat exchanger and condenser-reboiler surfaces by providing such surfaces with nucleation sites for the formation of vapor bubbles. Methods of forming such nucleation sites typically involve working the surface to provide cavities or channels therein, or providing a surface with a porous coating. Examples, of such improved boiling surfaces are given in, for example, U.S. Pat. Nos. 3,384,154, 3,457,990 and Re-issue 30,077 and U.K. Patent Application No. 2 155 612 A.
In conventional condenser-reboilers, flow of liquid oxygen through its respective exchange passages is by virtue of the head of liquid oxygen in which the condenser-reboiler is partially or totally immersed. In practice, a rise in the local boiling temperature is associated with the head of liquid oxygen, the boiling temperature rising from 0.5 to 1 degree K per metre depth of liquid. We have discovered that the boiling heat transfer co-efficient of a heat transfer surface is increased by forming a falling film of liquefied gas over the heat transfer surface. We have also found that the boiling heat transfer coefficient is further increased when the heat transfer surface has a multitude of nucleation sites for the formation of vapor bubbles.